Finishes are conventionally applied to leathers to impart protective coatings thereto, to level the color, and to minimize natural defects in the leather skins or hides. At the present time such finishes are generally applied by swabbing (brushing) and/or spraying the finish coatings upon the leathers; in some limited cases, as in the application of polyurethane coatings for patent leathers or for other effects, the finish coatings may be applied by flow-coating operations. In all such instances, sufficient finish is applied to the leather substrate to provide a continuous film upon the leather epidermis.
Typically, between two and six passes are required on conventional leather finishing equipment to deposit the desired continuous film on the leather substrates. Moreover, the leather may be plated between passes to facilitate the film-forming process and smooth out the sand-like surface formed by spray application of the finish. Each pass of the leathers to be finished through conventional finishing equipment subjects the same to pre-wetting and subsequent forced heat drying. Such operations tend to subject the collagen fibers to successive swelling and contraction, and impair the ultimate hand of the leather product.
Skins or hides employed in the manufacture of leathers differ from one another, not only in regard to the number, type and location of natural blemishes (scratches, scars, coarse grain, etc.) but also with respect to the character and quality of the skins or hides themselves. Different sections of any given animal skin exhibit varying absorption of leather finishes, and hence take on differing shades. Moreover, leather made from a well nourished, healthy animal has different absorptive characteristics than that made from an animal which is not as healthy or which may have been undernourished. Finish coatings are unevenly absorbed by such substrates. Thus, open cuts or bites on the leather epidermis absorb more finish and therefore darken relative to unblemished areas. Similarly, a well-nourished skin exhibits finish receptivity distinct from one not as well nourished. The application of finishes of uniform shades to varying leathers (or even to a single skin) is, therefore, exceedingly difficult to achieve employing conventional leather finishing operations.
Various procedures have also been proposed for laminating finish coatings to leather surfaces, particularly split leathers. Coatings thus applied have included various lacquers and, in more recent years, polyurethane finishes. See Poschel U.S. Pat. No. 1,955,562 granted Apr. 17, 1934; and Sutton U.S. Pat. No. 3,713,938 granted Jan. 20, 1973, British Pat. No. 1,268,763, and Das Leder 22, No. 5 (May 1971), pages 97-100, No. 7 (July 1971), pages 147-150 and No. 12 (December 1971), pages 269-276). Use of these prior techniques entails the application of some form of adhesive material prior to transferring the finish coating to the leather, a procedure which necessitates both the use of special equipment (coaters and drying ovens, for example) and imposes processing problems per se.
When, for example, an adhesive is sprayed or otherwise applied to the leather prior to transferring the finish thereto (see, for example, the cited "Das Leder" papers), it is important to control the particle size of the adhesive to thereby regulate its degree of penetration into the leather. Too great a penetration is said to result in poor adhesion of the finish, whereas insufficient penetration causes bubbling and wrinkling of the finish coating and distortion thereof. Moreover, leathers precoated with adhesive prior to application of the finish layers may block; to decrease the tack of the adhesives and thus overcome this problem the addition of non-thermoplastic binders thereto has been proposed. The addition of such material however, adversely affects the bond between the transfer finish and the leather substrate.
It is among the objects of the present invention to provide an improved method for finishing leather which may be efficiently and economically carried out, which requires only a single pass, and which provides more uniformly finished leathers than achieved in accordance with conventional leather finishing operations. A further object of the invention is to provide such a method employing finish coatings which may be transferred in the dry state without pre-treatment with any adhesives, tie-coats or the like, and which may thus be readily and simply practiced by tanners, shoe manufacturers or others without requiring the use of complex equipment or techniques such as employed in prior wet process transfer operations. These and other objects and advantages of the invention will be apparent from the following detailed description of preferred embodiments thereof, taken in connection with the accompanying drawing in which: